Specification Point 1.1:
Describe how the Dalton model of an atom has changed over time because of the discovery of subatomic particles.
- In 1803 John Dalton presented his atomic theory based on three key ideas:
- Matter is made of atoms which are tiny particles that cannot be created, destroyed or divided.
- Atoms of the same element are identical and atoms of different elements are different.
- Different atoms combine together to form new substances.
- At the time the theory was correct but as science developed some parts of Dalton’s theory were disproved.
- In 1897 physicist J.J. Thomson discovered the electron and using a cathode-ray tube conducted an experiment which identified the electron as a negatively charged subatomic particle, hence proving that atoms are divisible.
Diagram showing the electron beam deflecting towards the positive plate, proving electrons are negatively charged
- Based on his investigations Thomson proposed a model of the atom known as the plum pudding model which depicted negative electrons spread throughout soft globules of positively charged material.
Diagram showing the plum pudding model of the atom
- In 1909 Ernest Rutherford presented his model of the atom based on the gold foil experiment.
- Rutherford shot a beam of positively charged particles at a thin sheet of gold foil and, based on the plum pudding model, expected the particles to pass through the foil which most of them did.
- However a few were deflected and a few bounced directly back, which led him to postulate that most of an atom’s mass is concentrated in a region of space at the centre of the atom called the nucleus.
- In Rutherford’s model the atom consists mainly of empty space with the nucleus at the centre and the electrons orbiting in paths around the nucleus.
- This model was known as the planetary model of the atom.
Diagram showing Ernest Rutherford’s planetary model of the atom
Specification Point 1.2:
Describe the structure of an atom as a nucleus containing protons and neutrons, surrounded by electrons in shells.
- The central nucleus contains protons and neutrons only which are packed close together in a small region of space.
- Electrons move in the space outside the nucleus in orbits.
Diagram showing the atom and subatomic particles
The Subatomic Particles
Specification Point 1.3:
Recall the relative charge and relative mass of:
a) a proton
b) a neutron
c) an electron
The table below indicates the relative charge and mass of the subatomic particles.
The Neutral Atom
Specification Point 1.4:
Explain why atoms contain equal numbers of protons and electrons.
- Atoms are neutral and although they contain charged subatomic particles atoms are uncharged.
- Since the charge on a proton is +1 and on an electron is -1, the number of protons must be the same as the number of electrons for the charges to cancel each other out and produce a neutral atom.
Edexcel GCSE Chemistry Notes
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Morgan’s passion for the Periodic Table begun on his 10th birthday when he received his first Chemistry set. After studying the subject at university he went on to become a fully fledged Chemistry teacher, and now works in an international school in Madrid! In his spare time he helps create our fantastic resources to help you ace your exams.
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